In recent years, light sources such as light emitting diodes (LEDs) have been improved through technological advances in material and manufacturing processes. The LEDs possess characteristics such as a relatively high efficiency, a relatively long life, and vivid colors, and can be used in a variety of industries. One example is to use the LEDs to replace traditional incandescent bulbs in a vehicle lamp. Compared with traditional incandescent bulbs, the LEDs are lighter, compact, long-life, and energy-saving. Moreover, the response time of the LEDs is faster than that of the incandescent bulbs.
For some vehicles that are originally designed to be equipped with incandescent bulbs, there will be a problem if the incandescent bulbs are directly replaced by LEDs. FIG. 1 illustrates a conventional circuit 100 for using an incandescent bulb 102 in a vehicle. The incandescent bulb 102 is powered by a power source 108, e.g., a battery, via a power line 104. Under certain circumstances, a vehicle may need to perform a self-testing to examine whether the incandescent bulb 102 is turned on properly. A micro controlling unit (MCU) in the vehicle (not shown in FIG. 1) may generate a testing signal (usually a square wave signal) and apply the testing signal to the power line 104. A detecting circuit 106 monitors the voltage drop across the incandescent bulb 102. If a waveform of the testing signal has an amplitude greater than a predetermined level, the waveform can be detected by the detecting circuit 106. If the incandescent bulb 102 operates properly, the voltage drop across the incandescent bulb 102 is relatively small because the resistance of the filament in the incandescent bulb 102 is relatively small. Therefore, the waveform of the testing signal is not detected by the detecting circuit 106. If the incandescent bulb 102 is broken down (open circuit condition), the waveform of the testing signal can be detected by the detecting circuit 106 across the incandescent bulb 102. If the testing signal is detected by the detecting circuit 106, the detecting circuit 106 can determine that the incandescent bulb 102 is broken and remind the driver by turning on an indicator light on the dashboard.
FIG. 2 illustrates a conventional circuit 200 using LEDs to replace a traditional incandescent bulb in a vehicle. As shown in FIG. 2, an LED string 202 takes place of the incandescent bulb. The LED string 202 includes multiple LEDs connected in series. Generally, the resistance of the LED string 202 is greater than the resistance of an incandescent bulb. Therefore, when the micro controlling unit (not shown in FIG. 2) applies the testing signal on the power line 104, a waveform of the testing signal may be detected by the detecting circuit 106 across the LED string 202, even if the LED string 202 operates properly. The micro controlling unit may render an erred judgment. To prevent a false alarm, a dummy load, e.g., a resistor 204, is coupled to the LED string 202 in parallel. The resistor 204 can have a relatively small resistance such that the total resistance of the parallel-connected dummy load 204 and the LED string 202 is even smaller. By properly choosing the resistance of the resistor 204, the testing signal is not detected by the detecting circuit 106 across the LED string 202 such that the false alarm can be avoided. A drawback of this solution is that the resistor 204 will constantly consume power and generate heat if the vehicle lamp is turned on.